1. Field of the Invention
The present invention generally relates to the semiconductor device field, and more particularly, to a fin transistor structure and a method of fabricating the same.
2. Description of the Related Art
Fin transistor devices such as FinFETs are being in focus because of their good cut-off characteristics, excellent scalability, and compatibility with the conventional manufacturing processes. So far, conventional FinFETs are mainly categorized into two types: FinFETs formed on a Silicon On Insulator (SOI) substrate, and FinFETs formed on a bulk Si substrate (bulk-FinFET). The bulk-FinFET has many advantages over the FinFET on the SOI substrate, such as low cost, low body effect, low back-biased effect, and high heat transfer.
Document 1 (Tai-su Park el al., “Body-tied triple-gate NMOSFET fabrication using bulk Si wafer”, Solid-state Electronics 49(2005), 377-383) discloses a body-tied triple-gate NMOSFET fabricated by using a bulk Si wafer. FIG. 1 of this document illustrates a perspective view of this FET, and FIG. 2 shows the method of fabricating the FET in detail. As shown in FIGS. 1 and 2(f), a gate electrode of poly-silicon is formed across a fin where the channel of the semiconductor device is positioned. However, as clearly shown in FIG. 2(f), the channel has its bottom portion surrounded by Si3N4 and SiO2. As a result, the gate electrode cannot effectively control this portion. Thus, even in the off state, a current path may be formed between source and drain regions through the bottom portion of the channel, resulting in leakage current.
Document 2 (K. Okano el al., “Process Integration Technology and Device Characteristics of CMOS FinFET on Bulk Silicon Substrate with sub-10 nm Fin Width and 20 nm Gate Length”, IEDM 2005) discusses the above problem in more detail. Specifically, referring to FIG. 4 thereof, leakage current densities are shown for different portions of the fin. It can be seen that the leakage current density at the bottom of the channel is hundreds or even thousands of times greater than that at the channel region.
To solve the problem of leakage current, a punch through stop (PTS) structure may be introduced at the bottom of the channel so as to suppress the leakage current, as described in Document 2. In order to form such PTS structure at the bottom of the channel, high-energy ion implantation is often required. However, this will cause a broad distribution of the implanted dopants, and also high density of dopants in the channel region (referring to FIG. 5 of Document 2). Thus, such a structure is accompanied by large junction leakage and large junction capacitance.
Therefore, there is a need for a novel structure and a method for fabricating fin transistors, whereby it is possible to effectively reduce the leakage current at the bottom of the channel while maintaining the advantages of bulk-FinFETs, without causing high junction leakage and high junction capacitance.